Concerns that contamination from the Fukushima Daiichi plant had spread to Tokyo subsided on Friday after high levels of radiation recorded along a street in the city were linked to old bottles of radium stored beneath the floorboards of a nearby house.

Researchers had recorded radiation of 3.35 microsieverts per hour along a street in Setagaya ward, a higher level than in some parts of the 12-mile (20km) exclusion zone around the nuclear plant.

An investigation traced the contamination to several bottles that had been stored in a cardboard box beneath an empty house. The bottles recorded radiation levels in excess of those measurable on a low-dose radiation counter, said Setegaya's mayor, Nobuto Hosaka.

Science ministry officials believed the bottles contained radium-226, a radioactive material used in fluorescent paint on watch faces and in medical devices, the Yomiuri Shimbun said. Radiation levels inside the house dropped significantly after the bottles were placed inside a lead container, reports said.

The hourly reading in the Setagaya hotspot, located close to a nursery school, was equivalent to 17.6 millisieverts (mSv) a year, according to science ministry calculations, just below the 20mSv a year required to trigger an evacuation and more than 17 times the internationally recommended level for the general public.

Officials said the area had been cordoned off, adding that the contamination levels did not pose a threat to health.

The reading in Tokyo was taken a metre above the ground near a hedge, according to the public broadcaster NHK. Other spots along the same street showed lower readings.

Although this recent incident is not connected to the Fukushima disaster, the discovery comes amid concern that fallout from the plant may have spread over a much wider area than previously thought.

Kyodo News reported that a citizens' group detected levels as high as 5.82 microsieverts an hour in a park in the town of Funabashi, Chiba prefecture, 130 miles from Fukushima. That is five times higher than the highest levels recorded in the city since the 11 March disaster.

Earlier this week officials in Yokohama, just south of Tokyo, said they had found abnormally high levels of strontium-90 in sediment on the roof of a block of flats.

The radioactive isotope, which has a half-life of 29 years, can accumulate in the bones and cause bone cancer and leukaemia. In September officials in Yokohama said they had detected 40,200 becquerels of radioactive caesium per kilogramme of sediment collected from a roadside ditch.

The task of identifying how far the contamination has spread, and in what quantities, is proving difficult. Wind direction and topology can cause radiation to spread unevenly, and particles are more likely to gather in ditches and other places that accumulate dust and rainwater.

Setagaya ward officials said they would screen more than 250 other locations in the area over the next month. Radiation levels in the neighbourhood, which has a population of more than 840,000, have not dropped despite decontamination efforts.

Children have been warned to avoid the recently discovered hotspot, now blocked off by several plastic cones, on their way to and from school.

"I thought the reading must have been a mistake when I first saw it," Hosaka told the broadcaster TBS on Thursday. "Once we have confirmed the readings as high we will push ahead with decontamination efforts."